The following description of general background of the present invention makes reference to the appended drawing FIGS. 1 through 4 which show prior art systems. The combustion process of coal in power utility fired boilers produces two types of waste products; 1) ash particles that are small enough to be entrained in the flue gas referred to as fly ash, and 2) relatively large ash particles that overcome drag in the combustion gases and drop to the bottom of the boiler referred to as bottom ash. Typically, bottom ash is either collected in a water impoundment or in a dry bottom. Water impounded ash, referred to as wet bottom ash, is typically collected in individual water filled hoppers, as shown in FIG. 1 which illustrates a typical bottom ash-to-pond system 10, or in a closed loop recirculation system 26 shown in FIG. 2, or in a water filled trough with a submerged drag chain system 12 as shown in FIG. 3. In the system of FIG. 1, ash is discharged each shift in a batch process from hoppers 14 through bottom gate 16 on the side of the hoppers 14. Ash grinders 18 are provided to reduce ash particle size to less than about 3 in. (typically) to allow conveyance in a pipe as an ash/water slurry. The slurry is discharged into a storage pond 20 where the ash settles out over time. Surge tank 30 is provided to handle transient surges in the slurry flow. Numerous pumps 22 and valves 24 are provided for moving the slurry through system 10 (these elements are also shown in FIGS. 2, 3 and 4).
Closed loop recirculation system 26 shown in FIG. 2 is a modified form of system 10 and provides closed loop dewatering system and uses a settlement unit referred to by applicant as a “Hydrobin®” unit 28 as shown in FIG. 2. In the system 26 shown in FIG. 2, bottom ash 11 is discharged from hoppers 14 into the grinder 18 and is then pumped as an ash-water slurry to remotely located Hydrobin® dewatering bins 28 which provide a two-stage settling process necessary to clarify the water enough for recycling. Settled ash is drained of water through screens in the dewatering bins 28. Surge tank 30 and settling tank 32 handle the drained water and provide further clarification and separation of coal ash from the water. Clarified water is recycled back to convey the next batch of ash slurry. Dewatered ash slurry is hauled away from the plant site.
Systems and 10 and 26 are so-called wet sluicing systems which operate successfully but have a number of drawbacks, principally requiring large amounts of transport water that requires sophisticated treatment as well as significant capital expenditures.
The submerged mechanical drag conveyor system 12 (or submerged chain conveyor or “SCC”) is illustrated in FIGS. 3 and 4, and is typically applied to provide continuous ash removal. Bottom ash continuously falls into the SCC 12 through hopper discharge 42 and settles onto a chain-and-flight conveyor system referred to as a submerged drag chain unit 34. Unit 12 forms an open trough which is filled with water to quench the dry ash as it falls into the unit from the boiler. The chain of unit 34 moves continuously and carries away ash which is dewatered as it moves along an inclined section 36 and is transported via a conveyor 44 and into a bottom ash silo 38, and is later discharged into a truck to transport the material off-site. Make-up water is added to offset water loss with wet ash being removed from the system and due to evaporation. Mill reject hoppers 40 are provided to process such material which is directed onto chain conveyor inclined section 36 for processing along with the bottom ash slurry stream. The submerged drag chain conveyor unit 34 is positioned directly beneath the boiler ash hopper discharge 42. The boiler throat being rectangular shaped requires the orientation of the submerged drag chain conveyor unit 34 and boiler ash hopper discharge 42 to be substantially parallel to the major axis of the boiler throat. Another view of submerged drag chain conveyor unit 12 is shown in FIG. 4 which further illustrates the conveyor drive unit 46 and take-up unit 48 which provide proper conveyor chain tensioning. In this prior art system, one of the units 12 shown in FIGS. 3 and 4 is provided for each boiler ash hopper discharge 42.
The handling of ash from large-scale coal burning boilers is subject to ever increasingly stringent governmental regulations, including the US EPA's federal ELG (Effluent Limitations Guidelines) rules. These rules treat different forms of water streams found in bottom ash handling systems in different ways. For example, these rules preclude the discharge into the environment of ash transport water such as used in the pond system 10 shown in FIG. 1 and in the closed loop hydraulic system 26 shown in FIG. 2, and ash basins. Water streams not subject to these ELG requirements (presently) include quench water used in submerged chain conveyor systems 12 and other minor discharges. Retrofitting existing coal-fired boilers to modern ash handling system frequently involves a considerable capital expense. Operators of these systems will often decommission boilers in view of the significant expenses associated with retrofits.
With the above considerations in mind, boiler operators are often faced with difficult decisions regarding continuing the lifetime of existing installations. Installation of a conventional submerged drag chain system 12 as illustrated in FIG. 3 ordinarily requires removal of existing bottom ash hoppers 14 and replaced with a rectangular shape trough hopper 42 that can accept a continuous flow of ash. As mentioned previously, make-up water must be added to offset water loss. The water temperature is relatively high in these systems and therefore a cooling system is provided, such as through recirculation to a pond or installation of heat exchangers. Existing SCC systems 12 provide the benefits of not requiring transport water and the equipment cost is relatively low. In addition, maintenance and operating costs are relatively low as compared with wet sluicing systems. However, significant disadvantages are associated with the major reworking of the boiler mentioned above and the significant space requirements of such systems including orientation constraints. Since the system 12 is situated directly beneath the boiler without any isolation valves, a break in the SCC chain or other maintenance issue may require boiler shutdown in order to repair the fault.
This invention is related to embodiments of simplified submerged chain conveyor systems which are adaptable for retrofit applications which avoid the disadvantages mentioned previously. Several embodiments of the invention are illustrated and described herein.